Interaction Forces on Polyampholytic Hydrogel Gradient Surfaces.

Rational design and informed development of nontoxic antifouling coatings requires a thorough understanding of the interactions between surfaces and fouling species. With more complex antifouling materials, such as composites or zwitterionic polymers, there follows also a need for better characterization of the materials as such. To further the understanding of the antifouling properties of charge-balanced polymers, we explore the properties of layered polyelectrolytes and their interactions with charged surfaces. These polymers were prepared via self-initiated photografting and photopolymerization (SIPGP); on top of a uniform bottom layer of anionic poly(methacrylic acid) (PMAA), a cationic poly(2-dimethylaminoethyl methacrylate) (PDMAEMA) thickness gradient was formed. Infrared microscopy and imaging spectroscopic ellipsometry were used to characterize chemical composition and swelling of the combined layer. Direct force measurements by colloidal probe atomic force microscopy were performed to investigate the forces between the polymer gradients and charged probes. The swelling of PMAA and PDMAEMA are very different, with steric and electrostatic forces varying in a nontrivial manner along the gradient. The gradients can be tuned to form a protein-resistant charge-neutral region, and we demonstrate that this region, where both electrostatic and steric forces are small, is highly compressed and the origin of the protein resistance of this region is most likely an effect of strong hydration of charged residues at the surface, rather than swelling or bulk hydration of the polymer. In the highly swollen regions far from charge-neutrality, steric forces dominate the interactions between the probe and the polymer. In these regions, the SIPGP polymer has qualitative similarities with brushes, but we were unable to quantitatively describe the polymer as a brush, supporting previous data suggesting that these polymers are cross-linked.

pH-control of the protein resistance of thin hydrogel gradient films.

We report on the preparation and characterization of thin polyampholytic hydrogel gradient films permitting pH-controlled protein resistance via the regulation of surface charges. The hydrogel gradients are composed of cationic poly(2-aminoethyl methacrylate hydrochloride) (PAEMA), and anionic poly(2-carboxyethyl acrylate) (PCEA) layers, which are fabricated by self-initiated photografting and photopolymerization (SIPGP). Using a two-step UV exposure procedure, a polymer thickness gradient of one component is formed on top of a uniform layer of the oppositely charged polymer. The swelling of the gradient films in water and buffers at different pH were characterized by imaging spectroscopic ellipsometry. The surface charge distribution and steric interactions with the hydrogel gradients were recorded by direct force measurement with colloidal-probe atomic force microscopy. We demonstrate that formation of a charged polymer thickness gradient on top of a uniform layer of opposite charge can result in a region of charge-neutrality. This charge-neutral region is highly resistant to non-specific adsorption of proteins, and its location along the gradient can be controlled via the pH of the surrounding buffer. The pH-controlled protein adsorption and desorption was monitored in real-time by imaging surface plasmon resonance, while the corresponding redistribution of surface charge was confirmed by direct force measurements.

Polypeptide-guided assembly of conducting polymer nanocomposites.

A strategy for fabrication of electroactive nanocomposites with nanoscale organization, based on self-assembly, is reported. Gold nanoparticles are assembled by a polypeptide folding-dependent bridging. The polypeptides are further utilized to recruit and associate with a water soluble conducting polymer. The polymer is homogenously incorporated into the nanocomposite, forming conducting pathways which make the composite material highly conducting.

Lateral control of protein adsorption on charged polymer gradients.

This work describes the fabrication, characterization, and protein adsorption behavior of charged polymer gradients. The thin gradient films were fabricated by a two-step technique using UV-initiated free-radical polymerization in a reactor with a moving shutter. A homogeneous layer of cationic poly(2-aminoethyl methacrylate hydrochloride) was first formed, followed by a layer of oppositely charged poly(2-carboxyethyl acrylate) with a continuously increasing thickness. Adsorption from protein solutions as well as human blood plasma was investigated by imaging surface plasmon resonance and infrared microscopy. The results showed excessive protein adsorption in the areas where one of the polymers dominated the composition, while there was a clear minimum at an intermediate position of the gradient. The charge of the surface was estimated by direct force measurements and found to correlate well with the protein adsorption, showing the lowest net charge in the same area as the protein adsorption minimum. We therefore hypothesize that a combination of the charged polymers, in the right proportions, can result in a protein-resistant surface due to balanced charges.